Cut chicken into ½ inch cubes. Heat olive oil in an oven safe skillet over medium heat, (if you don’t have an oven safe skillet you can make the filling in a normal saucepan or frying pan and then transfer to a baking dish) add chicken to skillet. Cook until no longer pink in the middle. Remove chicken from skillet and set aside.
Add 1 tablespoon butter and diced onions to skillet. Once onions are soft, add 1 cup of chicken broth. Once chicken broth is warm remove a few tablespoons of it and mix it with the cornstarch in a separate bowl. Then add the cornstarch slurry into the pan with the chicken broth.
Stir in mixed vegetables and Greek yogurt. Let mixture cook until thick and vegetables are heated, add more chicken broth if the sauce is too thick or if it’s too thin you can add more cornstarch. Add in cooked chicken and season with dried sage, salt, and pepper to taste.
Preheat oven to 450 degrees. Stir together flour, baking powder, sugar, and salt. Mix in butter until you have a crumbly texture. Stir in milk until you have a soft dough.
Drop tablespoons of dough on top of your filling. Use the back of a spoon to spread dough across the top. Brush top of dough with beaten egg (you'll probably use about half of the egg). Bake for 12-14 minutes. Serve while still warm.
You can also double this recipe and bake it in a 9x13 pan.